1. Technical Field
The present invention relates to a test system having both an electric test function and an optical test function for testing a liquid crystal display device, such as a liquid crystal panel, and a test method therefor.
2. Prior Art
As is shown in a flowchart in FIG. 1, a liquid crystal panel is fabricated in a substrate forming process, an array forming process, a cell forming process and a module forming process. Generally, in this fabrication processing series, an electric test, called an array test, is conducted at the final stage of the array forming process, and a cell optical test is conducted at the final stage of the cell forming process. As the electric test in the array forming process, a TFT array test or an open short circuit test is commonly performed. According to the TFT array test, an electric charge writing voltage is applied to a data line and a voltage is applied to a scan line, so that electric charges are written to a pixel capacitor to be tested. When a predetermined period of time has elapsed, the electric charges at the test target are measured using a probe. Using the measured values of the electric charges, signal line short circuits, line breaks and defective pixels can be found. According to the open short circuit test, a voltage is applied alternatively to a scan line and a data line, and signal line short circuits, line breaks, and line intersection short circuits can be found.
For the optical test in the cell forming process, a liquid crystal panel wherein a cell is formed and into which liquid crystal is injected is turned on and the operational state of the panel is examined. Specifically, a drive signal for displaying an image pattern, such as a raster image, vertical strips, horizontal stripes or a box pattern, is transmitted to a data line terminal and a scan line terminal, and defective pixels, an even image color or an image contrast is visually examined.
The optical test is performed only for a liquid crystal panel that is regarded as good following the electric test. In other words, since a liquid crystal panel having poor electric characteristics would be eliminated by the electric test that is performed prior to the optical test, it would not be a target for the optical test. Theoretically, therefore, it can be ascertained that liquid crystal panel defects found by the optical test are the results of problems that occur during the cell forming process, such as dust entering a liquid crystal cell when it is sealed or the occurrence of scratches. However, when the reason for the failure of a defective panel found by the optical test is analyzed, it is found that the reason the failure occurred is not always dust or scratches, and the cause may be by the electrical characteristics of a pixel capacitor. It is felt that although originally electric characteristics are detected at the electric test, the reason that such a failure is found during the optical test is that it is caused by a device used for the test, the test method or differences in the testing conditions.
The pixel defect for an ON point or an OFF point that is to be examined at the optical test can be caused by a signal line short circuit, a line break or a pixel capacitor failure, or by the entry of dust or by scratches acquired during the cell forming process. However, since visually a defect caused by either failure has the same appearance, the cause of the defect can not be identified merely through visual observation. For this reason, both an electric testing device and an optical testing device must be employed to analyze a failure and to determine its cause, and such an analysis of a failure provides results that are very inadequate.